1. Field of the Invention
This invention relates to the heat-treatment of a thermoplastic ferroelectric copolymer of vinylidene fluoride and trifluoroethylene which results in the significant increase in the Curie temperature (heating) of the material, thereby increasing the thermal range over which the material is piezoelectric.
2. General Description of the Prior Art
It is a well known practice to construct transducers for ultrasonic nondestructive inspection devices with piezoelectric materials. It is also well known that random copolymers of vinylidene fluoride and trifluoroethylene, particularly within approximately the mole ratio stated in this invention, are employed and are being developed for use in such devices because of the piezoelectricity of these copolymers. These copolymers can also be used in pyroelectric sensors and devices. Such random copolymers are provided as random semicrystalline copolymers which can be shaped or formed into semicrystalline, essentially unoriented and unstretched, thermoplastic film or sheet or tubular-constructed product via such well known processing methods as extrusion, injection molding, compression molding and solvent casting. The materials so provided within the the approximate mole ratios of this invention contain, under ambient conditions, essentially a single crystalline phase whose structure is the ferroelectric all-trans chain conformation referred to in the prior art and herein as the Form-I or the .beta.-phase. The piezoelectric and thermal properties of the .beta.-phase are essentially the same as described in the article by K. Koga and H. Ohigashi, J. Appl. Phys., 59, (6), 2142, (1986). The .beta.-phase polar structure of the crystalline lattice confers significant piezoelectric properties on the copolymers after such copolymers have been electrically polarized.
As is well known, the term piezoelectric means the ability of a material to exchange electrical for mechanical energy and vice versa and the electromechanical response is believed to be essentially associated with dimensional changes during deformation or pressure oscillation. It is further known that these copolymers possess high electromechanical coupling factors (about 30%) that render them particularly useful for ultrasonic transducers (A. J. Lovinger, Japanese J. Appl. Phys., 24, Supplement 24-2, 18, (1985)).
The piezoelectric characteristic of such copolymer material exists when the material is in the ferroelectric state and, conversely, such piezoelectric characteristic does not exist to a commercially usable degree when the material is in the paraelectric state. It is well known that the copolymers of this invention undergo, upon heating, a first order Curie transition which effects a crystalline phase change from the .beta.-phase polar ferroelectric state to the nonpolar paraelectric state (T. Yagi, M. Tatemoto, J. Sako, Polymer J., 22, 209, (1980), and also T. Furukawa, G. E. Johnson, H. E. Bair, Y. Tajitsu, A. Chiba, E. Fukada, Ferroelectrics, 32, 61, (1981)). The Curie transition for these copolymers is well characterized, upon heating, by an endotherm which exhibits a temperature peak called the Curie temperature. Heating through the Curie transition will cause a copolymer which has been polarized, and is therefore highly piezoactive, to become depolarized, and therefore piezoinactive. It is well known to measure the phase change and the Curie temperature by a technique referred to in the prior art as differential scanning calorimetry, referred to herein as DSC.
When the paraelectric phase is cooled, the Curie transition is reversed and the ferroelectric phase is formed or reformed, as the case may be, accompanied by a characteristic exotherm and Curie temperature peak. The Curie temperature peak on cooling is lower than the Curie temperature peak on heating due to hysteresis, as is well known, and the difference may be as much as 50 degrees C. (A. Odajima, Ferroelectrics, 57, 159, (1984)).
As used herein, the term "Curie temperature (heating)" refers to the temperature peak on the endotherm, as determined by differential scanning calorimetry, DSC, measured during heating through the ferroelectric to paraelectric transition. As used herein, the term "Curie temperature (cooling)" refers to the temperature peak on the exotherm, as determined by differential scanning calorimetry, DSC, measured during cooling through the paraelectric to ferroelectric transition.
It is further well known that the endothermic Curie temperature (heating) increases as the mole ratio of vinylidene fluoride in the copolymers increases, being approximately 70 degrees C. at 65 mole percent vinylidene fluoride and approximately 130 degrees C. at 80 mole percent vinylidene fluoride, while the crystalline melting temperature of approximately 150 degrees C. remains relatively unchanged within a few degrees (K. Tashiro, K. Takano, M. Kobayashi, Y. Chatani, H. Tododoro, Polymer, 24, 199, (1983), A. J. Lovinger, T. Furukawa, G. T. Davis, M. G. Broadhurst, Ibid., 1233).
A problem with the copolymers of the prior art is that the Curie temperature (heating) of such copolymers is too low, thereby limiting the temperature range over which the material can be used and stored. The problem is obviously more acute with the copolymers of this invention which have the lower mole ratio vinylidene fluoride--and therefore a lower Curie temperature (heating)--where even a minimal potential increase is desirable. In addition, in the course of manufacture and assembly of piezoelectric derivative devices, it is often advantageous to expose assembly to a curing, sealing, soldering, electroding, etc. processing procedure whose heat generation may be sufficient to effect a depolarization of the copolymer with loss of piezoelectric activity. Further, it has been pointed out that from a device standpoint it would be attractive to be able to tailor the Curie temperature (heating) over a range of temperatures without requiring preparation of an increasing number of mole ratio compositions, (J. Green, B. L. Farmer, J. F. Rabolt, J. Appl. Phys., 60, 2690. (1986).
There is a need, therefore, for a copolymer material of the type described herein, which material has a significantly elevated Curie temperature (heating), thereby providing piezoelectric activity over a wider temperature range. There is a need, also, for a method for producing such material. The invention described herein provides a material and a method which satisfies these needs.
Prior art teaches that, for the polymer shaping methods, such as extrusion, injection, quench cooling of compression molded pieces or other methods for which rapid cooling is a final step, for one to obtain substantive piezoactivity, it is necessary and desirable to anneal the above produced materials for a short time. (H. Ohigashi, K. Koga, Japanese J. Appl. Phys., 21, L455-L457, (1982). Current art elaborates more fully on the useful application of such short-time annealing which is applied at a temperature (essentially 140 degrees C.) which is above the temperature region which coincides with the region in which its ferroelectric to paraelectric (Curie) transition occurs, but below the crystalline melting point. (K. Kimura, H. Ohigashi, Appl. Phys. Lett., 43, 834, (1983); U.S. Pat. No. 4,578,442 to H. Ohigashi et al.
One effect of such annealing is to reduce the Curie temperature (heating). Thus, K. Koga, H. Ohigashi, (Op. cit. (1986), report that the 74 mole percent vinylidene copolymer obtained by ice quenching from the melt had a Curie temperature (heating) of 129 degrees C., while after the annealing treatment the Curie temperature (heating) is 123 degrees C. An additional citation which shows that the above type of short-time annealing produces a reduction in Curie temperature (heating) is offered by Y. Oka, Y. Murata, N. Koizumi, Polymer J., 18, 417, (1986)). The random copolymer therein of 65 mole percent vinylidene fluoride and 35 mole percent trifluoroethylene showed that annealing at 120 degrees or 140 degrees C. for up to about one day reduced the Curie temperature (heating) over that obtained by fast quenching methods.
Similarly, the effect of such annealing is given by J. S. Green, B. L. Farmer, J. G. Rabolt (Op. cit.) Their data refers to a 60 mole percent vinylidene fluoride copolymer, a composition at the limits of that covered by this invention. In addition, processes that use a slow cooling rate from the melt temperature of the copolymer (as opposed to the rapid cooling processes mentioned above) also lower the Curie temperature (heating). In other words, the slow cooling is similar in effect to a rapid cooling followed by an anneal, as described above. (G. M. Stack, R. Y. Ting, Polymer Preprints, 27, (2) 161, (September 1986), Y. Oka, Y. Murata, N. Koizuma, (Op. cit.)).
As indicated above, the Curie temperature (heating) of each composition is influenced by its immediate prior heat-treatment. It is well known that the highest temperature peak in the DSC heating thermogram is associated with the melting point of the paraelectric phase, with the Curie temperature (heating) being the nearest peak to, and on the low temperature side, of the melting point peak. In order to obtain a reference value of Curie temperature (heating) for comparison purposes, it is useful to provide a thermal history which is the same for every sample. As used herein the term "constant thermal history" means the following procedure to establish a standard Curie temperature (heating) reference and a standard Curie temperature (cooling) reference value: Heating a sample at a rate of 10 degrees C./min. while recording the DSC thermogram curve generated; continuing the heating until the curve passes through the melting peak at approximately 150 degrees C.; continuing the heating at the same rate until the melt reaches 210 degrees C.; maintaining the melt at 210 degrees C. for 10 minutes; then, cooling at 10 degrees C./min., passing through the crystallization temperature exotherm and the paraelectric to ferroelectric transition exotherm; cooling 17 degrees C.; and finally reheating the sample to 210 degrees C. at 10 degrees C./min. through the ferroelectric to paraelectric Curie transition and paraelectric crystalline melting point transition.
The DSC heating curve recorded on the second heating indicates a reference Curie temperature (heating). As used herein the term "reference Curie temperature (heating)" means the Curie temperature (heating) as determined on a sample subject to the "constant thermal history" as defined hereinabove. The "reference Curie temperature (heating)" has an associated ferroelectric to paraelectric phase transition. Therefore, as used herein, the term "critical temperature range" means the temperature range within which the "reference Curie temperature (heating)" occurs and within which its associated ferroelectric to paraelectric phase transition occurs.
The DSC cooling curve recorded on the first cooling indicates a reference Curie temperature (cooling). As used herein, the term "reference Curie temperature (cooling)" means the Curie temperature (cooling) as determined on a sample subject to the "constant thermal history" as defined hereinabove. These reference Curie temperatures (heating) and (cooling) are within a few degrees of the Curie temperatures (heating) and (cooling) obtained by the annealing processes of the prior art described above, and therefore can be taken to represent the Curie temperatures (heating) and (cooling) of samples as if such samples were treated in accordance with the teachings of such prior and current art.
Another aspect of the current art teaches that short-time annealing of a poled sample at its Curie temperature (heating) causes the Curie temperature (heating) endotherm to separate into two peaks. One broader peak is below the original Curie temperature (heating) of the sample, and one sharper peak is above the original Curie temperature (heating) of the sample. The lower peak represents depolarized material and the upper peak represents the remaining polarized material. This dual peak occurrence is consistent with prior art teaching that, for an unpoled sample, its ferroelectric to paraelectric transition region is broader, and its associated Curie temperature (heating) is lower, than the transition region and Curie temperature heating) is on the originally poled sample, from which the unpoled sample is derived. This formation of dual peaks was only observed down to 8 degrees C. below the Curie temperature (heating) of the original sample.